(1) Technical Field of the Invention
The invention relates to an optical WDM transmission system including a plurality of first optical transceivers at a first end of an optical WDM transmission link which are to be tuned such that the individual center wavelength of each channel signal created by the first optical receivers lies within a dedicated optical channel of the optical WDM transmission system and/or is controlled such that the position of the center wavelength within the dedicated optical channel is optimized. Further, the invention relates to a suitable method for tuning a tunable optical transmitter in an optical WDM transmission system as well as to a central tuning device and a first optical transceiver for a respective optical WDM transmission system.
(2) Description Of Related Art
In most optical data transmission applications using optical wavelength division multiplexing (WDM), the wavelengths of the optical transmitters (mostly lasers) need to be locked to dedicated channel wavelengths, for example according to the ITU wavelength grid with optical frequencies spaced by 100, 50, or 25 GHz. For this purpose, wavelength lockers are available using optical filter devices, like etalons, which are periodic with the ITU frequency spacing. The known methods for locking the wavelength or optical frequency of a transmit signal to a dedicated channel frequency use a fixed relative position of the periodical optical transfer function of an optical filter device in order to lock the frequency of the transmitter unit to a fixed position of the respective transmission band of the filter. This fixed position usually corresponds to a center point between the minimum and the maximum point of the filter transfer characteristic.
The non pre-published prior European patent application 11 401 589 describes a method and device for locking the optical wavelength or frequency of a plurality of narrow-band optical channel transmit signals created by a plurality of optical transceivers to a dedicated optical channel frequency, the channel transmit signals having arbitrary channel frequency spacings, which allows the use of readily available optical filter devices having a transfer function with a different periodicity. Here, a common locking or tuning device is used which creates respective tuning control signals for the tunable optical transmitters. These tuning control signals are directly supplied to the respective optical transmitters. In order to create the tuning control signals, an optical WDM detection signal is tapped off from the optical WDM signal including all of the digital optical channel signals. If more than one digital optical channel signal is included in the optical WDM detection signal, the digital optical channel signals are amplitude-modulated with a given low modulation frequency (small as compared to the bitrate) and a given predetermined small modulation depth. Thus, the tapped-off optical WDM detection signal, which includes the amplitude-modulated digital optical channel signals, can be detected at the central tuning device in a phase-sensitive manner. This makes it possible to determine a tuning control information separately for each of the digital optical channel signals included in the optical WDM signal or optical WDM detection signal, respectively.
Using such a central tuning device saves cost as compared to conventional tunable optical transceivers which usually include a separate wavelength control and wavelength stabilization for each transceiver. The required components, like wavelength lockers, thermo-electric coolers (TEC) contribute to this cost. Additionally, the characterization of each transceiver module to determine the tuning parameters essentially contributes to the overall cost. However, this proposed use of a central tuning device requires to locate the central tuning device close to the optical transmitters to be controlled.